Laminar flow is achieved by reducing the magnitude of disturbances and instabilities in the boundary-layer. By keeping these fluctuations small, the nonlinear interactions leading to turbulence can be curtailed and/or delayed. Currently, the most robust methods for controlling the disturbance amplitudes are based on modifying the boundary-layer mean flow via airfoil geometry (i.e., by tailoring the pressure gradient, C.sub.p) or by applying surface suction.
Since modifications to the pressure gradient do not actively consume power, this approach has been termed "natural laminar flow". The successful application of this approach and attainment of drag reduction benefits has been demonstrated both theoretically and in testing for nominally two-dimensional boundary-layers. The main disadvantage of the natural laminar flow approach is that the modified C.sub.p distribution is generally unacceptable from an overall airplane performance point of view. For this reason, natural laminar flow is not frequently used for increasing the extent of laminar flow.
The use of suction has also been successfully tested to show improved laminar flow and reduced drag without the adverse restrictions on the C.sub.p distribution. However, the suction approach has its own shortcomings, including increased costs, added weight, and increased complexity of the overall flow-control system as compared to the baseline non-suction configuration. These shortcomings partially offset the performance savings. There are also potential performance penalties associated with suction applications, e.g., suction drag and increased roughness sensitivity due to thinner boundary-layers. Additionally, the porous suction surface can require increased maintenance.
It is also known to use a combination of suction and pressure gradient tailoring (termed "hybrid laminar flow control") to effectively achieve laminar flow with more practical C.sub.p distributions. While the overall performance of the aircraft is improved to acceptable levels, the hybrid laminar flow control approach still suffers the shortcomings of the suction system.
The application of surface air cooling (to below the adiabatic surface temperature) has also been theorized to be an effective flow control technique. The general theory predicts that cooling of an airflow surface to lower than the adiabatic surface temperature will cool the passing boundary-layer which in turn will slow the development and growth of instabilities. Conceived surface cooling techniques, however, are thought to be impractical for large surface areas such as those in a large commercial transport. Because of this, the idea of surface cooling is not exploited in current aircraft configurations.
The beneficial effects of surface cooling have also been theorized to occur by application of local heat to a stable upstream region of the boundary-layer. In theory, the heated upstream boundary-layer then encounters a cooler downstream surface to result in a net change in temperature decrease experienced by the boundary-layer that is similar to the net change in temperature achieved by simply cooling the downstream surface. This approach was demonstrated experimentally at TsAGI and at I.T.A.M. in Russia during the mid-to-late 1980's. Specifically, the results showed that increased laminar flow could be achieved by localized heating in the leading-edge region of a flat plate. (See for example, Dovgal, A. V., Levchenko, V. Ya. and Timofeev, V. A. (1990) "Boundary layer control by a local heating of the wall," from: IUTAM Laminar-Turbulent Transition, eds. D. Arnal and R. Michel, Springer-Verlag, pp. 113-121). One of the problems in applying this alternative technique to airfoils has been the loss of performance benefit after only a relatively short period of time due to the transfer of heat from the boundary-layer flow to the cooler surface downstream. As heat is transferred from the boundary-layer flow, the surface temperature rises and the relative temperature difference between the flow and the surface diminishes. This reduces the stabilizing effect on the boundary-layer and eventually terminates the laminar-flow benefit.
In summary, the drag reduction benefits of having laminar airflow have been known for many years, however, there are few economically viable laminar airflow control systems available. The general problem has been that the increased costs required to achieve sustained laminar flow substantially erodes the potential benefits. Usually, the laminar flow control system does improve laminar flow over an aerodynamic surface (e.g., wing, nacelle, vertical tail, etc.) and improve overall aircraft performance, but the benefits of the system are more than offset by the increased costs in manufacturing, maintenance, aircraft weight, design complexity, etc. Thus, a need exists for a laminar flow control means that is low cost and low maintenance. The ideal system would further have minimum impact on the weight and configuration complexity of the aircraft.